Major renovation of the field conversion functionality. Made more
robust, with better performance and using a lot less code.
So far, only conversion between singlefield and multifield of
the same base type, and conversion to and from SoSFString and
SoMFString has been implemented.

* removed the "cleanClass" and "clean" methods -- almost all of them were
empty, and we should anyway use a better technique for doing cleanup at
exit -- ANSI C or POSIX provides a hook we could use.
* moved common code in node classes from templates to macros.
* added class definition and code skeletons for all nodekit related classes.
* misc other minor fixes.